Salt maintenance in the internal fluids of a fish is accomplished by special biological salt pumps located in the gills. The biological salt pumps are configured for salt elimination for salt water fish. The biological salt pumps are configured for salt retention for fresh water fish.
The skin of fish is water permutable. A fresh water fish absorbs water through the skin and eliminates excess water through the kidneys. A salt water fish loses water through the skin and is forced to drink salt water.
When a salt water fish is placed in fresh water the salt water fish will have difficulties maintaining adequate internal salt balance. When a fresh water fish is placed in salt water the fresh water fish will have difficulties eliminating excess salt. Both fish when placed in non-native salt level environments will soon die without appropriate intervention.
A special configuration exists for the biological salt pumps in the gills of live-bearing prenatal fry. The biological salt pumps are configured for both salt retention and elimination. This is due to the salinity of the amniotic fluid surrounding the prenatal fry.
In the normal case, if the fry are birthed in fresh water, the elimination biological salt pumps are sequentially disabled. In the exception case, if the fry are birthed in water matching the salinity of the amniotic fluid, the elimination biological salt pumps are not disabled but continue to operate. With the aid of natural occurring hormones, salinity of the water can now safely be increased to the salinity of sea water.
Once the fry reach adulthood they breed true. The adult fish and their descendants can now breed and flourish in salt water without further interventions.